Photo-fabrication, which is currently in vogue for a precision microfabrication technique, is the generic name for techniques in which various precision parts such as semiconductor packages are manufactured by applying a photosensitive resin composition to the surface of a material to be processed to form a coating film, patterning the coating film by photolithographic techniques, and performing electroforming, which generally involves chemical etching, electrolytic etching, and/or electroplating, using the patterned coating film as a mask.
Known conventional photo-acid generators for use in curing cationically polymerizable compounds such as epoxy compounds by the action of active energy rays such as light, electron beams, or X-rays include triarylsulfonium salts (Patent Document 1), phenacylsulfonium salts having a naphthalene skeleton (Patent Document 2), dialkylbenzylsulfonium salts (Patent Document 3), and sulfonium salts having an introduced thioxanthone skeleton (Patent Document 4).
At present, the i-line with a wavelength of 365 nm is widely used as exposure light in photolithographic processes using photoresists. Some reasons for this are that medium- or high-pressure mercury lamps, which are available as irradiation light sources at a low cost and have high emission intensity, can be used, and LED lamps having an emission wavelength in the i-line region (360 nm to 390 nm) are currently becoming widespread. It is therefore considered that a need for photo-acid generators having high sensitivity to the i-line will further increase in the future.
However, some existing photo-acid generators such as triarylsulfonium salts (Patent Document 1), phenacylsulfonium salts having a naphthalene skeleton (Patent Document 2), and dialkylbenzylsulfonium salts (Patent Document 3) have low sensitivity to the i-line and therefore need to be used in combination with a sensitizer so that the sensitivity can be increased. Sulfonium salts having an introduced thioxanthone skeleton (Patent Document 4) also have a problem that their i-line absorbance is too high so that light may fail to reach a deep portion at the time of curing a thick film, which causes curing failure.
Moreover, in recent years, as the size of electronic devices has further decreased, high-density packaging of semiconductor packages has progressed, in which the packaging density has been increased based on multi-pin thin film packaging or package downsizing, or two- or three-dimensional packaging techniques using flip-chip method. Connection terminals formed by such high-density packaging techniques include projecting electrodes (mounted terminals) such as bumps projecting on packages; and metal posts for connecting mounted terminals to rewiring parts extending from peripheral terminals on a wafer, which are placed with high precision on a substrate.
Materials for use in such high-precision photo-fabrication include positive photosensitive resin compositions containing a naphthoquinonediazide group-containing compound (see Patent Document 5). However, such materials have a problem that they have low sensitivity to the i-line (365 nm), which is an exposure light source generally used in photolithographic processes.
A chemically amplified positive resist composition in which an oxime sulfonate compound is used as an acid generator is proposed as a photosensitive resin composition for reducing the above problem (see Patent Document 6). When this composition is irradiated with (exposed to) radiations, an acid is generated from the photo-acid generator, and heat treatment after the exposure facilitates the diffusion of the acid and the acid-catalyzed reaction, so that the base resin in the resin composition undergoes a change in solubility in an alkali. Such a composition is called a positive photoresist, because the base resin, which is insoluble in alkali before the exposure, is made alkali-soluble. However, this resist composition has low storage stability and therefore needs complicated storage-temperature control and has a practical problem.
There are also known chemically amplified positive resist compositions in which a triarylsulfonium salt compound is used as a photo-acid generator (Non-Patent Documents 1 and 2 and Patent Documents 7 and 8). Although they have high storage stability, they are not considered to have sufficient sensitivity to the i-line.
Conventionally, polyimide-based resins, which are excellent in heat resistance, mechanical properties, and the like, have been widely used to form surface protecting films, interlayer dielectric films, and others for use in semiconductor devices in electronics.
There are also proposed various photosensitive polyimide-based resins having photosensitivity for an improvement in the precision of film formation by high integration of semiconductor devices (Patent Documents 9, 10, and 11).
Such compositions need a cyclization process in which heat treatment is performed at high temperature for imidization, and therefore, film loss may occur after curing, or process conditions such as temperature control conditions are complicated.
To reduce such problems, there are also proposed photosensitive resin compositions produced with an alkali-soluble resin having a phenolic hydroxyl group (Patent Documents 12 and 13).
Such compositions are negative photoresist compositions, in which an acid is generated from a photo-acid generator by exposure to light and facilitates the reaction between a crosslinking agent and a base resin so that the resin becomes insoluble in a developing solution. However, such compositions contain a triazine-type photo-acid generator and therefore have a problem that the acid generated from the triazine-based photo-acid generator is hydrochloric or hydrobromic acid, which is volatile and pollutes facilities.
Conventional sulfonium salt-based photo-acid generators also have a problem that they have low sensitivity to the i-line (365 nm) which is an exposure light source generally used in photolithographic processes.